Acid-alkali process for the preparation of cellulose fibers



Patented Jan. 13, 1953 ACID-ALKALI PROCESS FOR THE PREPARA- TION FCELLULOSE FIBERS James D. Atkinson, Jr., and Linwood Nicholas Rogers,Memphis, Tenn., assignors to Buckeye Cotton Oil Company, Memphis, Tenn,a corporation of Ohio No Drawing. Application June 30, 1947,

v Serial N0. 758,234

3 Claims.

This invention relates to the preparation of cellulose fibers frombagasse. Consisting principally of the fibrous residue of sugar, caneafter extraction of the juice, bagasse is a waste material available inenormous quantities at sugar mills, and is commonly used as fuel at themills for lack of a better method of disposal. It is the principalobject of the invention to provide a process whereby the cellulosefibers in bagasse may be economically liberated and further purified toproduce a high grade pulp suitable for use in paper making, in themanufacture of cellulose products and derivatives such as viscose andcuprammonium rayon, also cellulose ethers and esters, and for use invarious other fields which have heretofore been supplied solely by pulpderived from other and more expensive sources.

Extensive efiorts have been made over many years to devise asatisfactory process for producing cellulose from bagasse. Treatment ofbagasse both with nitric acid and with caustic soda is now employed butthese efforts have achieved noting more than the production on a smallscale of pulp for low grades of wrapping paper and paper board. So faras we are aware, no pulp produced by prior methods isv suitable for usein high class paper, and even a good quality of paper stock is far fromsufiiciently pure for the rayon trade. By means of the presentinvention, however, we are enabled to obtain from bagasse a cellulosepulp having certain qualities and characteristics comparable to thoseexhibited by pulp derived from cotton linters, and suitable for use notonly in the manufacture of the best grade of paper, but in viscoserayon. Furthermore, these results are achieved by employing a novelsequence of processing steps which involve no appreciable increase incost over methods heretofore proposed for the treatment of bagasse. i

In general, our process is based on conventional nitric acid processing,which comprises treatment with nitric acid, followed by alkalinedigestion and bleaching. It dilfers, however, in several respects whichwe have found to be essential to the production of high grade pulp. Thusthe present process may consist essentially of the following steps,taken in the order named:

1. The bagasse, as received from the sugar mill, is mechanicallyprocessed to eifect reduction in size, and mud is removed by washing. g

2. The finely divided bagasse is treated with nitric acid at an elevatedtemperature, the acid drained, and the material washed.

3. The material is subjected to alkaline digestion at elevatedtemperature.

4. The resulting pulp is then violently agitated to break up fiberbundles and washed.

5. A second treatment with alkali at elevated temperature is applied.

6. After washing and screening, the pulp is chlorinated, and againwashed.

7. The chlorinated pulp is steeped in alkali without substantialheating. I

8. The pulp is rebleached and soured in accordance with knownprocedures, formed into sheets, and dried.

While the selection of appropriate conditions for each processing stepis of importance in determining the character of the final product,these conditions are, in general, not critical and may be varied withinfairly wide ranges. The sequence of operations is, however, quiteimportant; of special significance is the treatment which immediatelyfollows the nitration, namely steps numbered 3 to 5 above, which arebelieved to be responsible in large part for the excellent qualities ofthe cellulose produced by our method. The steps which follow therepeated alkaline extraction are, for the most part, conventional, butthe effect of this later processing is probably greatly altered by theunique alkaline treatment and agitation to which the pulp is subjectedfollowing the nitric acid treatment. The several steps of the processwill now be describedin more detail.

1. Mechanical disintegration and washing.- The purpose of this step isto reduce the bagasse to smaller size to facilitate handling and toaccelerate the subsequent chemical reaction. The removal of mud andother entrained substances is more readily eifected whenthe bagasse isin. a

more finely divided state than as customarily received from the sugarmill.

Any conventional means may be employed to subdivide the bagasse, such asa hammer mill or a beater; the operation should be continued until theparticles are reduced to a length of not more than one-half inch andpreferably smaller.

2. Nitric acid treatment.This is a common step in previously proposedmethods of liberating fibers from bagasse, and exerts a threefold actionon the material: (a) Part of the encrusting material, probably lignin,is nitrated. (b) Other encrusting materials, such as xylans, arehydrolyzed by the acid. (0) Oxidation of part of the encrustants iseffected, as evidenced by evolution of NO.

We prefer to employ a dilute nitric acid solution, for instance 2%, inthe ratio of 9 parts of treating solution to one part of bagasse byweight, ordinarily sufii-cien-t to insure complete immersion of thesolids, and the reaction may be allowed to continue for about one hourat a temperature between 80 C. and 110 C., for instance 95 C. Strongeracid is unnecessary, and therefore uneconomic-a1, as is longer time orhigher temperature. The action does no-tstart well at temperatures below90 (3., but once initiated is slightly exothermic for about minutes,after which applied heat may agin be necessary to maintain the selectedtemperature. Used acid seems to be more efiective than fresh acid, andmay therefore be employed in somewhat less concentration, for instance1.5%. Thus it is desirable to drain and wash the acid fromthe treatedbagasse, to introduce a sufficient amount of fresh acid to restore thestrength of the solution, and to re-use the same for the treatment ofsucceed ing batches of bagasse. 4 1

3. First alkaline extraction-In this treatment the humins, nitrocompounds, and other. materials such as hemi-celluloses are eitherwholly or partly dissolved.

The preferredalkaline agent is sodium hye droxide, which may be employedin the form of a dilute solution, for instance 1% to 3%, thereaetionbeing continued for about one hour at a temperature between 80and 110 C., preferably about 95 C. .As in various othersteps of theprocess, we find it convenient to use a 9:1 ratiov of solution to solidmaterial. Other alkaline agents may be employed to replac all or part ofthe caustic soda, in most instances with less economy when used inquantity sufficient to effect results comparable to those achieved "bycaustic soda, for instance alkaline carbonates and sulphites, or any ofthe various alkaline agents heretofore suggested for use intheliberation of fibers from cellulosic materials. The conditions of thereaction are not critical and may be varied over a considerable range,as iseviden-ced by the earlier literature on this subject.

-It is in the course of this step that the bagasse isfirst truly pulped,the fiber bundles being reduced in size so as to approach individualfiber dimensions 4. Mechanical agitation.The mechanical disintegrationof the pulp at this stage, followed by further alkaline treatment, isimportant to successful practice of the invention. The effect of theagitation is to break up further the fiber bundles in order that thecaustic treatment may be more effective and complete, whereby the sodasoluble value may be lowered without any accompanying degradation of thecellulose. The apparatus employed in this step may vary widely and maybe selected from the many devices capable ofviolently agitating slurry.Following agitation, the material is preferably washed 5. Secondalkaline extraction-Further treatment with alkali at this point isnecessary in order to complete the treatment initiated in step; 3. This,step alsotends to lower the soda soluble value and to reduce theconsumption of chemicals in the subsequent bleaching operation byremoving easily oxidizable hemi-cellulosis from the pulp. l a r As instep 3, conventional alkaline agents may be employed, the-preferredagent being sodium hydroxide, of which 1% concentration is usuallysufficient, although solutions of less or greater strength may beemployed. A convenient liquid tosolids ratio"ls9:1, and the reaction maybe carried out under conditions similar to those used in the firstalkaline digestion for a period of aboutone hour. The conditions are notcritical andfsubstantial vari-ations,' suggested the 7 screen of about0.008 inch slots, to insure that 4 literature for caustic treatment ofpulp, may be followed.

At the conclusion of the treatment, the pulp may be agitated somewhatmore to effect complete reduction of the fiber bundles to suitable size,and is washed. The amount of dirt in the pulp is reduced by screeningthrough a pulp the bleach will not be wasted on improperly purifiedpulp.

6. Chlorination-The principal effect of this processing stepis to effectchlorination of remaining encrustants, especially those of the lignintype which are readily susceptible. Conventional agents and processingconditions may be employed, such as are common in the paper and relatedindustries. Thus we may employ anacid hypochlorite bleach for one hourat'room temperature (about 25- 0.), or we may use a straightchlorination step with direct chlorineat high density. The details ofthis processing step form no part of the instant invention. Followingthe chlorination, the pulp is washed.

7. Alkali steep.Treatment at this stage is preferably carried out atroom temperature (25 C.) since the low temperature steeping promotesswelling of the fibers and renders the remaining encrustants moreaccessible to further treatment. Chlorinated lignins and hemicellulosesare also removed. We prefer to employ sodium hydroxide, but otheralkaline agents known to be useful in this field may be used. Solutionsof caustic soda of concentration varying from 5% to 14% maybesatisfactorily employed, the actual concentration being substantiallyless, due to the presence of water in the pulp. For instance, we prefer9% sodium hydroxide to facilitate reuse as hereinafter mentioned; theactual concentration is then about 5.6%. Actual concentrations of theorder of 10% to 12% of caustic solution tend to result in pilly pulp,due to easy mercerization of the pulp, and such high concentrations areunnecessary in any event. Operation at reduced temperatures effects someslight improvement of the fiber, but is hardly worth the added expense.Treatment for about one hour is usually entirely adequate.

The treated pulp is washed, and the wash liquors may be re-used in thedigesting steps 3 and 5 by suitable adjustment of the concentration.

8. Rebleaching.This step is also one which is practiced in accordancewith established procedures and with conventional agents such aschlorine or an agent yielding chlorine. It is preferably carried out inthe presence of a slight excess of alkali, as is usually practiced inknown processes. 7

Following the rebleach, the pulp is soured and washed in accordance withconventional procedure, and is then formed in sheets and dried, as iscustomary in the pulp industry.

While we prefer to carry out steps 6, 7, and 8 in the order named, thissequence of operations is subject to considerable variation. Thus thestrong alkaline steep of step 7 may precede step 6, or may be omittedaltogether for the manufacture of paper, with resultant increase inyield. Again, the alkaline steep may be employed both before and aftereach bleach step, or may follow either the chlorination or rebleachsteps. As hereinbefore pointed out, these stages of the process are ofmuch less significance in producing high grade pulp than are thesuccessive mild alkaline treatments and mechanical agitation whichimmediately follow the treatment with nitric acid.

Highly successful result have been achieved by processing bagasse in themanner now briefly outlined and given by way of example.

Year-old Louisiana bagasse is washed and slightly disintegrated in aheater. The washed bagasse is then cooked in 2% nitric acid at 9:1 ratioat 95 C. for one hour. The material is washed, and then cooked in 2%sodium hydroxide at 9:1 ratio at 95 C. for one hour. The fiber is thendisintegrated by mechanical agitation, washed, and again cooked insodium hydroxide under the same conditions. After washing, furthermechanical disintegration, and screening, the pulp is steeped in 9%sodium hydroxide at a ratio of 4.521 at 20 C. for one hour. The pulp iswashed, bleached with chlorine solution at 9:1 ratio and under acidconditions, and again steeped for two hours in 9% sodium hydroxide at 20C. Following further washing and rebleaching with a hypochloritesolution with excess alkalinity, the pulp is soured, washed, formed intosheets, and air-dried to 7% moisture.

When only one caustic steep is employed in the foregoing example, thechlorine bleaching and the succeeding operations are conductedimmediately following the step of screening.

It will be observed that each reaction step may be carried out by simpleimmersion of th solid material in the treating solution, and thatelevated pressures are not required. Thus the method lends itselfreadily either to cont nuous, semi-continuous, or batch processing withunusual economy.

Sheet pulp produced by our process is somewhat more dense than standardcotton linters pulp, and while not like board, is somewhat stifier thancotton linters pulp. It swells considerably more than cotton linterspulp during steeping for viscose. The alpha cellulose content is high,often exceeding 96.5%. Filtration of viscose pre ared from the pulp isreasonably good, and far exceeds that of pulp liberated from ba asse byprior methods.

The pulp from bagasse may be used in blends of bulbs for various puroses. For example. in banknote paper and bond paper it improves therattle": it will increase the wei ht of board: and will also add o acityto a sheet. since the fibers are so fine. The alpha cellulose willdepend on the exact process used, which will depend in turn on the enduse for which the pulp is intended.

Havin thus described the invention. what is claimed as new and desiredto be secured by Letters Patent is:

1. A process for liberating and purif in fibers from bagasse whichconsists in immersing the ba asse. prior to alkal ne treatment thereof.in dilute nitric acid solution having a concentration of about 2% atelevated temperature between 80" C. and 110 C. for a eriod of about onehour, removing the acid solution and washing the bagasse, thereafterimmersing the bagasse in dilute sodium hydroxide solution having aconcentration of about 1% to 3% at about 95 C. for a period of about onehour to reduce the size of fiber bundles and form a pulp, mechanicallyand violently agitating the pulp before 7 washing to break up the fiberbundles. thereafter washing the pulp, again immersing the pulp in dilutesodium hydroxide solution at about C. for a period of about one hour,chlorinating the pulp to effect bleaching and removal of encrustants,steeping the pulp in the cold in 5% to 14% sodium hydroxide solution fora period of about one hour, and rebleaching the pulp in a hypochloritesolution with excess alkalinity.

2. A process for liberating and purifying fibers from bagasse whichconsists in immersing the bagasse, prior to alkaline treatment thereofin 2% nitric acid solution at about 95 C. for a pe riod of about onehour, removing the acid solution and washing the bagasse, thereafterimmersing the bagasse in 2% sodium hydroxide solution at about 95 C. fora period of about one hour to reduce the size of fiber bundles and forma pulp, mechanically and violently agitating the pulp before washing tobreak up the fiber bundles, thereafter washing the pulp, again immersingthe pulp in 2% sodium hydroxide solution at about 95 C. for a period ofabout one hour. further mechanically agitating and Washing the pulp,steeping the pulp in 9% sodium hydroxide solution without substantialheating, and thereafter bleaching the pulp.

3. A process for liberating and purifying fibers from bagasse whichconsists in immersing the bagasse, prior to alkaline treatment thereof,in 2% nitric acid solution at about 95 C. for a period of about onehour, removing the acid solution and washing the bagasse, thereafterimmersing the bagasse in 2% sodium hydroxide solution at about 95 C. fora period of about one hour to reduce the size of fiber bundles and forma pulp, mechanically and violently agitating the pulp to break up thefiber bundles, again immersing the pulp in 2% sodium hydroxide solutionat about 95 C. for a period of about one hour, further mechanicallyagitating and washing the pulp, and thereafter bleaching the pulp.

JAMES D. ATKINSON, JR. LINWOOD NICHOLAS ROGERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,690,954 Spencer Nov. 6, 19281,829,852 Darling Nov. 3, 1931 1,840,162 Heimann et al. Jan. 5, 19321,987,195 Kipper Jan. 8, 1935 2,013,042 Foster Sept. 3, 1935 2,183,643Dreyfus Dec. 19, 1939 2,301,314 Montonna Nov. 10, 1942 FOREIGN PATENTSNumber Country Date 14,988 Great Britain of 1884 OTHER REFERENCESIndustrial and Engineering Chemistry, vol. 30, pages 1192 and 1198(1938).

Industrial and Engineering Chemistry, November 1932, pages 1249 to 1254.

Industrial and Engineering Chemistry, December 1934, pages 1339 to 1341.

Paper World, April 1939, pages 41 to 48.

Paper World, June 1939, pages 335 to 343.

1. A PROCESS FOR LIBERATING AND PURIFYING FIBERS FROM BAGASSE WHICHCONSISTS IN IMMERSING THE BAGASSE, PRIOR TO ALKALINE TREATMENT THEREOF,IN DILUTE NITRIC ACID SOLUTION HAVING A CONCENTRATION OF ABOUT 2% ATELEVATED TEMPERATURE BETWEEN 80* C. AND 110* C. FOR A PERIOD OF ABOUTONE HOUR, REMOVING THE ACID SOLUTION AND WASHING THE BAGASSE, THEREAFTERIMMERSING THE BAGASSE IN DILUTE SODIUM HYDROXIDE SOLUTION HAVING ACONCENTRATION OF ABOUT 1% TO 3% AT ABOUT 95* C. FOR A PERIOD OF ABOUTONE HOUR TO REDUCE THE SIZE OF FIBER BUNDLES AND FORM A PULP,MECHANICALLY AND VIOLENTLY AGITATING THE PULP BEFORE WASHING TO BREAK UPTHE FIBER BUNDLES, THEREAFTER WASHING THE PULP, AGAIN IMMERSING THE PULPIN DILUTE SODIUM HYDROXIDE SOLUTION AT ABOUT 95* C. FOR A PERIOD OFABOUT ONE HOUR, CHLORINATING THE PULP TO EFFECT BLEACHING AND REMOVAL OFENCRUSTANTS, STEEPING THE PULP IN THE COLD IN 5% TO 14% SODIUM HYDROXIDESOLUTION FOR A PERIOD OF ABOUT ONE HOUR, AND REBLEACHING THE PULP IN AHYPOCHLORITE SOLUTION WITH EXCESS ALKALINITY.